HONEYWELL E M AUTOMATIC CONTROL for - Homestead€¦ · tremendous advances in equipment, system design, and application. In this edition, microprocessor controls are shown in most

ENGINEERING MANUAL OF AUTOMATIC CONTROL i

HONEYWELL

ENGINEERING MANUAL of

AUTOMATICCONTROL forCOMMERCIAL BUILDINGS

ENGINEERING MANUAL OF AUTOMATIC CONTROLii

Copyright 1934, 1940, 1953, 1988, 1991 and 1997 by Honeywell Inc.

All rights reserved. This manual or portions thereof may not be reporducedin any form without permission of Honeywell Inc.

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ENGINEERING MANUAL OF AUTOMATIC CONTROL iii

FOREWORD

The Minneapolis Honeywell Regulator Company published the first edition of the Engineering Manual ofAutomatic Control in l934. The manual quickly became the standard textbook for the commercial buildingcontrols industry. Subsequent editions have enjoyed even greater success in colleges, universities, and contractorand consulting engineering offices throughout the world.

Since the original 1934 edition, the building control industry has experienced dramatic change and madetremendous advances in equipment, system design, and application. In this edition, microprocessor controls areshown in most of the control applications rather than pneumatic, electric, or electronic to reflect the trends inindustry today. Consideration of configuration, functionality, and integration plays a significant role in thedesign of building control systems.

Through the years Honeywell has been dedicated to assisting consulting engineers and architects in theapplication of automatic controls to heating, ventilating, and air conditioning systems. This manual is an outgrowthof that dedication. Our end user customers, the building owners and operators, will ultimately benefit from theefficiently designed systems resulting from the contents of this manual.

All of this manual’s original sections have been updated and enhanced to include the latest developments incontrol technology. A new section has been added on indoor air quality and information on district heating hasbeen added to the Chiller, Boiler, and Distribution System Control Applications Section.

This twenty-first edition of the Engineering Manual of Automatic Control is our contribution to ensure thatwe continue to satisfy our customer’s requirements. The contributions and encouragement received from previoususers are gratefully acknowledged. Further suggestions will be most welcome.

Minneapolis, MinnesotaOctober, 1997

KEVIN GILLIGANPresident, H&BC Solutions and Services

ENGINEERING MANUAL OF AUTOMATIC CONTROLiv

ENGINEERING MANUAL OF AUTOMATIC CONTROL v

PREFACE

The purpose of this manual is to provide the reader with a fundamental understanding of controls and howthey are applied to the many parts of heating, ventilating, and air conditioning systems in commercial buildings.

Many aspects of control are presented including air handling units, terminal units, chillers, boilers, buildingairflow, water and steam distribution systems, smoke management, and indoor air quality. Control fundamentals,theory, and types of controls provide background for application of controls to heating, ventilating, and airconditioning systems. Discussions of pneumatic, electric, electronic, and digital controls illustrate that applicationsmay use one or more of several different control methods. Engineering data such as equipment sizing, use ofpsychrometric charts, and conversion formulas supplement and support the control information. To enhanceunderstanding, definitions of terms are provided within individual sections. For maximum usability, each sectionof this manual is available as a separate, self-contained document.

Building management systems have evolved into a major consideration for the control engineer when evaluatinga total heating, ventilating, and air conditioning system design. In response to this consideration, the basics ofbuilding management systems configuration are presented.

The control recommendations in this manual are general in nature and are not the basis for any specific job orinstallation. Control systems are furnished according to the plans and specifications prepared by the controlengineer. In many instances there is more than one control solution. Professional expertise and judgment arerequired for the design of a control system. This manual is not a substitute for such expertise and judgment.Always consult a licensed engineer for advice on designing control systems.

It is hoped that the scope of information in this manual will provide the readers with the tools to expand theirknowledge base and help develop sound approaches to automatic control.

ENGINEERING MANUAL OF AUTOMATIC CONTROLvi

ENGINEERING MANUAL OF AUTOMATIC CONTROL vii

CONTENTS

Foreward ............................................................................................................................................................. iii

Preface ................................................................................................................................................................ v

Control System Fundamentals ............................................................................................ 1

Control Fundamentals ....................................................................................................................................... 3Introduction .......................................................................................... 5Definitions ............................................................................................ 5HVAC System Characteristics ............................................................. 8Control System Characteristics ........................................................... 15Control System Components .............................................................. 30Characteristics And Attributes Of Control Methods ............................. 35

Psychrometric Chart Fundamentals ................................................................................................................ 37Introduction .......................................................................................... 38Definitions ............................................................................................ 38Description of the Psychrometric Chart ............................................... 39The Abridged Psychrometric Chart ..................................................... 40Examples of Air Mixing Process .......................................................... 42Air Conditioning Processes ................................................................. 43Humidifying Process............................................................................ 44ASHRAE Psychrometric Chart ............................................................ 53

Pneumatic Control Fundamentals .................................................................................................................... 57Introduction .......................................................................................... 59Definitions ............................................................................................ 59Abbreviations ....................................................................................... 60Symbols ............................................................................................... 61Basic Pneumatic Control System ........................................................ 61Air Supply Equipment .......................................................................... 65Thermostats ........................................................................................ 69Controllers ........................................................................................... 70Sensor-Controller Systems ................................................................. 72Actuators and Final Control Elements ................................................. 74Relays and Switches ........................................................................... 77Pneumatic Control Combinations ........................................................ 84Pneumatic Centeralization .................................................................. 89Pneumatic Control System Example ................................................... 90

Electric Control Fundamentals ......................................................................................................................... 95Introduction .......................................................................................... 97Definitions ............................................................................................ 97How Electric Control Circuits Classified .............................................. 99Series 40 Control Circuits.................................................................... 100Series 80 Control Circuits.................................................................... 102Series 60 Two-Position Control Circuits ............................................... 103Series 60 Floating Control Circuits ...................................................... 106Series 90 Control Circuits.................................................................... 107Motor Control Circuits .......................................................................... 114

ENGINEERING MANUAL of

AUTOMATICCONTROL

ENGINEERING MANUAL OF AUTOMATIC CONTROLviii

Electronic Control Fundamentals ..................................................................................................................... 119Introduction .......................................................................................... 120Definitions............................................................................................ 120Typical System .................................................................................... 122Components ........................................................................................ 122Electtonic Controller Fundamentals .................................................... 129Typical System Application .................................................................. 130

Microprocessor-Based/DDC Fundamentals .................................................................................................... 131Introduction .......................................................................................... 133Definitions............................................................................................ 133Background ......................................................................................... 134Advantages ......................................................................................... 134Controller Configuration ...................................................................... 135Types of Controllers ............................................................................. 136Controller Software .............................................................................. 137Controller Programming ...................................................................... 142Typical Applications ............................................................................. 145

Indoor Air Quality Fundamentals ..................................................................................................................... 149Introduction .......................................................................................... 151Definitions............................................................................................ 151Abbreviations ....................................................................................... 153Indoor Air Quality Concerns ................................................................ 154Indoor Air Quality Control Applications ................................................ 164Bibliography ......................................................................................... 170

Smoke Management Fundamentals ................................................................................................................. 171Introduction .......................................................................................... 172Definitions............................................................................................ 172Objectives ............................................................................................ 173Design Considerations ........................................................................ 173Design Principles ................................................................................ 175Control Applications ............................................................................ 178Acceptance Testing ............................................................................. 181Leakage Rated Dampers .................................................................... 181Bibliography ......................................................................................... 182

Building Management System Fundamentals ................................................................................................. 183Introduction .......................................................................................... 184Definitions............................................................................................ 184Background ......................................................................................... 185System Configurations ........................................................................ 186System Functions ................................................................................ 189Integration of Other Systems............................................................... 197

ENGINEERING MANUAL OF AUTOMATIC CONTROL ix

Control System Applications ............................................................................................... 199

Air Handling System Control Applications ...................................................................................................... 201Introduction .......................................................................................... 203Abbreviations ....................................................................................... 203Requirements For Effective Control .................................................... 204Applications-General ........................................................................... 206Valve and Damper Selection ............................................................... 207Symbols ............................................................................................... 208Ventilation Control Processes ............................................................. 209Fixed Quantity of Outdoor Air Control ................................................. 211Heating Control Processes.................................................................. 223Preheat Control Processes ................................................................. 228Humidification Control Process ........................................................... 235Cooling Control Processes .................................................................. 236Dehumidification Control Processes ................................................... 243Heating System Control process ......................................................... 246Year-Round System Control processes .............................................. 248ASHRAE Psychrometric Charts .......................................................... 261

Building Airflow System Control Applications ............................................................................................... 263Introduction .......................................................................................... 265Definitions ............................................................................................ 265Airflow Control Fundamentals ............................................................. 267Airflow Control Applications ................................................................. 281References .......................................................................................... 292

Chiller, Boiler, and Distribution System Control Applications ....................................................................... 293Introduction .......................................................................................... 297Abbreviations....................................................................................... 297Definitions............................................................................................ 297Symbols ............................................................................................... 298Chiller System Control ......................................................................... 299Boiler System Control .......................................................................... 329Hot And Chilled Water Distribution Systems Control ........................... 337High Temperature Water Heating System Control .............................. 376District Heating Applications ................................................................ 382

Individual Room Control Applications ............................................................................................................ 399Introduction .......................................................................................... 401Unitary Equipment Control .................................................................. 412Hot Water Plant Considerations .......................................................... 428

ENGINEERING MANUAL OF AUTOMATIC CONTROLx

Engineering Information ....................................................................................................... 429

Valve Selection and Sizing ................................................................................................................................ 431Introduction .......................................................................................... 432Definitions............................................................................................ 432Valve Selection .................................................................................... 436Valve Sizing ......................................................................................... 441

Damper Selection and Sizing ............................................................................................................................ 451Introduction .......................................................................................... 453Definitions............................................................................................ 453Damper Selection ................................................................................ 454Damper Sizing ..................................................................................... 463Damper Pressure Drop ....................................................................... 468Damper Applications ........................................................................... 469

General Engineering Data ................................................................................................................................. 471Introduction .......................................................................................... 472Weather Data ...................................................................................... 472Conversion Formulas And Tables ........................................................ 475Electrical Data ..................................................................................... 482Properties Of Saturated Steam Data................................................... 488Airflow Data ......................................................................................... 489Moisture Content Of Air Data .............................................................. 491

Index ....................................................................................................................................... 494

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

1

CONTROLSYSTEMS

FUNDMENTALS

Contents

Introduction ............................................................................................................ 5

Definitions ............................................................................................................ 5

HVAC System Characteristics ............................................................................................................ 8General ................................................................................................ 8Heating ................................................................................................ 9General ................................................................................................ 9Heating Equipment .............................................................................. 10Cooling ................................................................................................ 11General ................................................................................................ 11Cooling Equipment .............................................................................. 12Dehumidification .................................................................................. 12Humidification ...................................................................................... 13Ventilation ............................................................................................ 13Filtration............................................................................................... 14

Control System Characteristics ............................................................................................................ 15Controlled Variables ............................................................................ 15Control Loop ........................................................................................ 15Control Methods .................................................................................. 16

General ........................................................................................... 16Analog And Digital Control .............................................................. 16

Control Modes ..................................................................................... 17Two-Position Control ....................................................................... 17

General ....................................................................................... 17Basic Two-Position Control ......................................................... 17Timed Two-Position Control ........................................................ 18

Step Control .................................................................................... 19Floating Control ............................................................................... 20Proportional Control ........................................................................ 21

General ....................................................................................... 21Compensation Control ................................................................ 22

Proportional-Integral (Pi) Control .................................................... 23Proportional-Integral-Derivative (Pid) Control ................................. 25Enhanced Proportional-Integral-Derivative (epid) Control .............. 25Adaptive Control ............................................................................. 26

Process Characteristics ....................................................................... 26

ControlFundamentals



4

Load ................................................................................................ 26Lag .................................................................................................. 27General ........................................................................................... 27Measurement Lag ........................................................................... 27Capacitance .................................................................................... 28Resistance ...................................................................................... 29Dead Time ....................................................................................... 29

Control Application Guidelines ............................................................ 29

Control System Components ............................................................................................................ 30Sensing Elements ............................................................................... 30

Temperature Sensing Elements ...................................................... 30Pressure Sensing Elements ............................................................ 31Moisture Sensing Elements ............................................................ 32Flow Sensors .................................................................................. 32Proof-Of-Operation Sensors ........................................................... 33

Transducers ........................................................................................ 33Controllers ........................................................................................... 33Actuators ............................................................................................. 33Auxiliary Equipment ............................................................................. 34

Characteristics And Attributes Of Control Methods .............................................................................................. 35



5

INTRODUCTION

This section describes heating, ventilating, and airconditioning (HVAC) systems and discusses characteristics andcomponents of automatic control systems. Cross-referencesare made to sections that provide more detailed information.

A correctly designed HVAC control system can provide acomfortable environment for occupants, optimize energy costand consumption, improve employee productivity, facilitateefficient manufacturing, control smoke in the event of a fire,and support the operation of computer and telecommunicationsequipment. Controls are essential to the proper operation ofthe system and should be considered as early in the designprocess as possible.

Properly applied automatic controls ensure that a correctlydesigned HVAC system will maintain a comfortableenvironment and perform economically under a wide range ofoperating conditions. Automatic controls regulate HVACsystem output in response to varying indoor and outdoorconditions to maintain general comfort conditions in officeareas and provide narrow temperature and humidity limitswhere required in production areas for product quality.

Automatic controls can optimize HVAC system operation.They can adjust temperatures and pressures automatically toreduce demand when spaces are unoccupied and regulateheating and cooling to provide comfort conditions whilelimiting energy usage. Limit controls ensure safe operation ofHVAC system equipment and prevent injury to personnel anddamage to the system. Examples of limit controls are low-limit temperature controllers which help prevent water coilsor heat exchangers from freezing and flow sensors for safeoperation of some equipment (e.g., chillers). In the event of afire, controlled air distribution can provide smoke-freeevacuation passages, and smoke detection in ducts can closedampers to prevent the spread of smoke and toxic gases.

HVAC control systems can also be integrated with securityaccess control systems, fire alarm systems, lighting controlsystems, and building and facility management systems tofurther optimize building comfort, safety, and efficiency.

DEFINITIONS

The following terms are used in this manual. Figure 1 at theend of this list illustrates a typical control loop with thecomponents identified using terms from this list.

Analog: Continuously variable (e.g., a faucet controlling waterfrom off to full flow).

Automatic control system: A system that reacts to a changeor imbalance in the variable it controls by adjustingother variables to restore the system to the desiredbalance.

Algorithm: A calculation method that produces a controloutput by operating on an error signal or a time seriesof error signals.

Compensation control: A process of automatically adjustingthe setpoint of a given controller to compensate forchanges in a second measured variable (e.g., outdoorair temperature). For example, the hot deck setpointis normally reset upward as the outdoor airtemperature decreases. Also called “reset control”.

Control agent: The medium in which the manipulated variableexists. In a steam heating system, the control agent isthe steam and the manipulated variable is the flow ofthe steam.

Control point: The actual value of the controlled variable(setpoint plus or minus offset).

Controlled medium: The medium in which the controlledvariable exists. In a space temperature control system,the controlled variable is the space temperature andthe controlled medium is the air within the space.

Controlled Variable: The quantity or condition that ismeasured and controlled.

Controller: A device that senses changes in the controlledvariable (or receives input from a remote sensor) andderives the proper correction output.

Corrective action: Control action that results in a change ofthe manipulated variable. Initiated when thecontrolled variable deviates from setpoint.

Cycle: One complete execution of a repeatable process. Inbasic heating operation, a cycle comprises one onperiod and one off period in a two-position controlsystem.

Cycling: A periodic change in the controlled variable fromone value to another. Out-of-control analog cyclingis called “hunting”. Too frequent on-off cycling iscalled “short cycling”. Short cycling can harm electricmotors, fans, and compressors.

Cycling rate: The number of cycles completed per time unit,typically cycles per hour for a heating or coolingsystem. The inverse of the length of the period of thecycle.



6

Deadband: A range of the controlled variable in which nocorrective action is taken by the controlled systemand no energy is used. See also “zero energy band”.

Deviation: The difference between the setpoint and the valueof the controlled variable at any moment. Also called“offset”.

DDC: Direct Digital Control. See also Digital and Digitalcontrol.

Digital: A series of on and off pulses arranged to conveyinformation. Morse code is an early example.Processors (computers) operate using digitallanguage.

Digital control: A control loop in which a microprocessor-based controller directly controls equipment basedon sensor inputs and setpoint parameters. Theprogrammed control sequence determines the outputto the equipment.

Droop: A sustained deviation between the control point andthe setpoint in a two-position control system causedby a change in the heating or cooling load.

Enhanced proportional-integral-derivative (EPID) control:A control algorithm that enhances the standard PIDalgorithm by allowing the designer to enter a startupoutput value and error ramp duration in addition tothe gains and setpoints. These additional parametersare configured so that at startup the PID output variessmoothly to the control point with negligibleovershoot or undershoot.

Electric control: A control circuit that operates on line or lowvoltage and uses a mechanical means, such as atemperature-sensitive bimetal or bellows, to performcontrol functions, such as actuating a switch orpositioning a potentiometer. The controller signalusually operates or positions an electric actuator ormay switch an electrical load directly or through arelay.

Electronic control: A control circuit that operates on lowvoltage and uses solid-state components to amplifyinput signals and perform control functions, such asoperating a relay or providing an output signal toposition an actuator. The controller usually furnishesfixed control routines based on the logic of the solid-state components.

Final control element: A device such as a valve or damperthat acts to change the value of the manipulatedvariable. Positioned by an actuator.

Hunting: See Cycling.

Lag: A delay in the effect of a changed condition at one pointin the system, or some other condition to which it is

related. Also, the delay in response of the sensingelement of a control due to the time required for thesensing element to sense a change in the sensedvariable.

Load: In a heating or cooling system, the heat transfer thatthe system will be called upon to provide. Also, thework that the system must perform.

Manipulated variable: The quantity or condition regulatedby the automatic control system to cause the desiredchange in the controlled variable.

Measured variable: A variable that is measured and may becontrolled (e.g., discharge air is measured andcontrolled, outdoor air is only measured).

Microprocessor-based control: A control circuit that operateson low voltage and uses a microprocessor to performlogic and control functions, such as operating a relayor providing an output signal to position an actuator.Electronic devices are primarily used as sensors. Thecontroller often furnishes flexible DDC and energymanagement control routines.

Modulating: An action that adjusts by minute increments anddecrements.

Offset: A sustained deviation between the control point andthe setpoint of a proportional control system understable operating conditions.

On/off control: A simple two-position control system in whichthe device being controlled is either full on or full offwith no intermediate operating positions available.Also called “two-position control”.

Pneumatic control: A control circuit that operates on airpressure and uses a mechanical means, such as atemperature-sensitive bimetal or bellows, to performcontrol functions, such as actuating a nozzle andflapper or a switching relay. The controller outputusually operates or positions a pneumatic actuator,although relays and switches are often in the circuit.

Process: A general term that describes a change in a measurablevariable (e.g., the mixing of return and outdoor airstreams in a mixed-air control loop and heat transferbetween cold water and hot air in a cooling coil).Usually considered separately from the sensingelement, control element, and controller.

Proportional band: In a proportional controller, the controlpoint range through which the controlled variablemust pass to move the final control element throughits full operating range. Expressed in percent ofprimary sensor span. Commonly used equivalents are“throttling range” and “modulating range”, usuallyexpressed in a quantity of engineering units (degreesof temperature).



7

SETPOINT

60

0

130

190

RESET SCHEDULE

HWSETPOINT

OA TEMPERATURE

160

159

148

AUTO

41

INPUT

OUTPUT

30

PERCENTOPEN

VALVE

STEAM

FLOW

OUTDOORAIR

OUTDOORAIR

CONTROLPOINT

HOT WATERRETURN

HOT WATERSUPPLY

HOT WATERSUPPLY

TEMPERATURECONTROLLED

MEDIUM

CONTROLLEDVARIABLE

MEASUREDVARIABLE

MEASUREDVARIABLE

SETPOINT

ALGORITHM INCONTROLLER

FINAL CONTROLELEMENT

CONTROLAGENT

MANIPULATEDVARIABLE

M10510

Proportional control: A control algorithm or method in whichthe final control element moves to a positionproportional to the deviation of the value of thecontrolled variable from the setpoint.

Proportional-Integral (PI) control: A control algorithm thatcombines the proportional (proportional response)and integral (reset response) control algorithms. Resetresponse tends to correct the offset resulting fromproportional control. Also called “proportional-plus-reset” or “two-mode” control.

Proportional-Integral-Derivative (PID) control: A controlalgorithm that enhances the PI control algorithm byadding a component that is proportional to the rate ofchange (derivative) of the deviation of the controlledvariable. Compensates for system dynamics andallows faster control response. Also called “three-mode” or “rate-reset” control.

Reset Control: See Compensation control.

Sensing element: A device or component that measures thevalue of a variable.

Setpoint: The value at which the controller is set (e.g., thedesired room temperature set on a thermostat). Thedesired control point.

Short cycling: See Cycling.

Step control: Control method in which a multiple-switchassembly sequentially switches equipment (e.g.,electric heat, multiple chillers) as the controller inputvaries through the proportional band. Step controllersmay be actuator driven, electronic, or directlyactivated by the sensed medium (e.g., pressure,temperature).

Throttling range: In a proportional controller, the control pointrange through which the controlled variable must passto move the final control element through its fulloperating range. Expressed in values of the controlledvariable (e.g., degrees Fahrenheit, percent relativehumidity, pounds per square inch). Also called“proportional band”. In a proportional roomthermostat, the temperature change required to drivethe manipulated variable from full off to full on.

Time constant: The time required for a dynamic component,such as a sensor, or a control system to reach 63.2percent of the total response to an instantaneous (or“step”) change to its input. Typically used to judgethe responsiveness of the component or system.

Two-position control: See on/off control.

Zero energy band: An energy conservation technique thatallows temperatures to float between selected settings,thereby preventing the consumption of heating orcooling energy while the temperature is in this range.

Zoning: The practice of dividing a building into sections forheating and cooling control so that one controller issufficient to determine the heating and cooling

requirements for the section.

Fig. 1. Typical Control Loop.



8

HVAC SYSTEM CHARACTERISTICS

Figure 2 shows how an HVAC system may be distributed ina small commercial building. The system control panel, boilers,motors, pumps, and chillers are often located on the lower level.The cooling tower is typically located on the roof. Throughoutthe building are ductwork, fans, dampers, coils, air filters,heating units, and variable air volume (VAV) units anddiffusers. Larger buildings often have separate systems forgroups of floors or areas of the building.

Fig. 2. Typical HVAC System in a Small Building.

The control system for a commercial building comprisesmany control loops and can be divided into central system andlocal- or zone-control loops. For maximum comfort andefficiency, all control loops should be tied together to shareinformation and system commands using a buildingmanagement system. Refer to the Building ManagementSystem Fundamentals section of this manual.

The basic control loops in a central air handling system canbe classified as shown in Table 1.

Depending on the system, other controls may be requiredfor optimum performance. Local or zone controls depend onthe type of terminal units used.

DAMPER

AIR FILTER

COOLING COIL

FAN

CHILLER

PUMP

COOLINGTOWER HEATING

UNIT

DUCTWORK

VAV BOXDIFFUSER

BOILERCONTROLPANEL

M10506

GENERAL

An HVAC system is designed according to capacityrequirements, an acceptable combination of first cost andoperating costs, system reliability, and available equipmentspace.



9

ControlLoop Classification Description

Ventilation Basic Coordinates operation of the outdoor, return, and exhaust air dampers to maintainthe proper amount of ventilation air. Low-temperature protection is often required.

Better Measures and controls the volume of outdoor air to provide the proper mix ofoutdoor and return air under varying indoor conditions (essential in variable airvolume systems). Low-temperature protection may be required.

Cooling Chiller control Maintains chiller discharge water at preset temperature or resets temperatureaccording to demand.

Cooling towercontrol

Controls cooling tower fans to provide the coolest water practical under existingwet bulb temperature conditions.

Water coil control Adjusts chilled water flow to maintain temperature.

Direct expansion(DX) systemcontrol

Cycles compressor or DX coil solenoid valves to maintain temperature. Ifcompressor is unloading type, cylinders are unloaded as required to maintaintemperature.

Fan Basic Turns on supply and return fans during occupied periods and cycles them asrequired during unoccupied periods.

Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces systemoperating cost and improves performance (essential for variable air volumesystems).

Heating Coil control Adjusts water or steam flow or electric heat to maintain temperature.

Boiler control Operates burner to maintain proper discharge steam pressure or water temperature.For maximum efficiency in a hot water system, water temperature should be resetas a function of demand or outdoor temperature.

Table 1. Functions of Central HVAC Control Loops.

HEATING

GENERAL

Building heat loss occurs mainly through transmission,infiltration/exfiltration, and ventilation (Fig. 3).

ROOF20°FTRANSMISSION

VENTILATION DUCT

EXFILTRATION

DOORWINDOW

PREVAILINGWINDS

INFILTRATION

70°F

C2701

Fig. 3. Heat Loss from a Building.

The heating capacity required for a building depends on thedesign temperature, the quantity of outdoor air used, and thephysical activity of the occupants. Prevailing winds affect therate of heat loss and the degree of infiltration. The heatingsystem must be sized to heat the building at the coldest outdoortemperature the building is likely to experience (outdoor designtemperature).

Transmission is the process by which energy enters or leavesa space through exterior surfaces. The rate of energytransmission is calculated by subtracting the outdoortemperature from the indoor temperature and multiplying theresult by the heat transfer coefficient of the surface materials.The rate of transmission varies with the thickness andconstruction of the exterior surfaces but is calculated the sameway for all exterior surfaces:

Energy Transmission per

Unit Area and Unit Time = (TIN - T

OUT) x HTC

Where:T

IN= indoor temperature

TOUT

= outdoor temperatureHTC = heat transfer coefficient

= Btu

Unit Time x Unit Area x Unit TemperaturHTC



10

Infiltration is the process by which outdoor air enters abuilding through walls, cracks around doors and windows, andopen doors due to the difference between indoor and outdoorair pressures. The pressure differential is the result oftemperature difference and air intake or exhaust caused by fanoperation. Heat loss due to infiltration is a function oftemperature difference and volume of air moved. Exfiltrationis the process by which air leaves a building (e.g., throughwalls and cracks around doors and windows) and carries heatwith it. Infiltration and exfiltration can occur at the same time.

Ventilation brings in fresh outdoor air that may requireheating. As with heat loss from infiltration and exfiltration,heat loss from ventilation is a function of the temperaturedifference and the volume of air brought into the building orexhausted.

HEATING EQUIPMENT

Selecting the proper heating equipment depends on manyfactors, including cost and availability of fuels, building sizeand use, climate, and initial and operating cost trade-offs.Primary sources of heat include gas, oil, wood, coal, electrical,and solar energy. Sometimes a combination of sources is mosteconomical. Boilers are typically fueled by gas and may havethe option of switching to oil during periods of high demand.Solar heat can be used as an alternate or supplementary sourcewith any type of fuel.

Figure 4 shows an air handling system with a hot water coil.A similar control scheme would apply to a steam coil. If steamor hot water is chosen to distribute the heat energy, high-efficiency boilers may be used to reduce life-cycle cost. Watergenerally is used more often than steam to transmit heat energyfrom the boiler to the coils or terminal units, because waterrequires fewer safety measures and is typically more efficient,especially in mild climates.

THERMOSTAT

HOT WATERSUPPLY

VALVE

DISCHARGEAIR

FAN

HOT WATERRETURN C2702

Fig. 4. System Using Heating Coil.

An air handling system provides heat by moving an airstream across a coil containing a heating medium, across anelectric heating coil, or through a furnace. Unit heaters (Fig.5) are typically used in shops, storage areas, stairwells, anddocks. Panel heaters (Fig. 6) are typically used for heatingfloors and are usually installed in a slab or floor structure, butmay be installed in a wall or ceiling.

C2703

UNIT HEATER

COIL

FAN

STEAM ORHOT WATERSUPPLY

CONDENSATEOR HOT WATERRETURN

STEAM TRAP(IF STEAM SUPPLY)

Fig. 5. Typical Unit Heater.

C3035

DISCHARGE AIR

WALL

OUTDOORAIR

MIXINGDAMPERS

RETURNAIR

COOLING COIL

DRAIN PAN

HEATING COIL

FAN

Fig. 6. Panel Heaters.

Unit ventilators (Fig. 7) are used in classrooms and mayinclude both a heating and a cooling coil. Convection heaters(Fig. 8) are used for perimeter heating and in entries andcorridors. Infrared heaters (Fig. 9) are typically used for spotheating in large areas (e.g., aircraft hangers, stadiums).

HOT WATERSUPPLY

HOT WATERRETURN

GRID PANEL

HOT WATERSUPPLY

HOT WATERRETURN

SERPENTINE PANEL

C2704

Fig. 7. Unit Ventilator.



11

Fig. 8. Convection Heater.

WARM AIR

FINNED TUBE

RETURN AIR

FLOORSUPPLY

RETURN

TO OTHERHEATING UNITS

FROM OTHERHEATING UNITS

C2705

REFLECTOR

INFRAREDSOURCE

C2706

RADIANT HEAT

Fig. 9. Infrared Heater.

In mild climates, heat can be provided by a coil in the centralair handling system or by a heat pump. Heat pumps have theadvantage of switching between heating and cooling modesas required. Rooftop units provide packaged heating andcooling. Heating in a rooftop unit is usually by a gas- or oil-fired furnace or an electric heat coil. Steam and hot water coilsare available as well. Perimeter heat is often required in colderclimates, particularly under large windows.

A heat pump uses standard refrigeration components and areversing valve to provide both heating and cooling within thesame unit. In the heating mode, the flow of refrigerant throughthe coils is reversed to deliver heat from a heat source to theconditioned space. When a heat pump is used to exchange heatfrom the interior of a building to the perimeter, no additionalheat source is needed.

A heat-recovery system is often used in buildings where asignificant quantity of outdoor air is used. Several types ofheat-recovery systems are available including heat pumps,runaround systems, rotary heat exchangers, and heat pipes.

In a runaround system, coils are installed in the outdoor airsupply duct and the exhaust air duct. A pump circulates themedium (water or glycol) between the coils so that mediumheated by the exhaust air preheats the outdoor air entering thesystem.

A rotary heat exchanger is a large wheel filled with metalmesh. One half of the wheel is in the outdoor air intake andthe other half, in the exhaust air duct. As the wheel rotates, themetal mesh absorbs heat from the exhaust air and dissipates itin the intake air.

A heat pipe is a long, sealed, finned tube charged with arefrigerant. The tube is tilted slightly with one end in theoutdoor air intake and the other end in the exhaust air. In aheating application, the refrigerant vaporizes at the lower end

in the warm exhaust air, and the vapor rises toward the higherend in the cool outdoor air, where it gives up the heat ofvaporization and condenses. A wick carries the liquidrefrigerant back to the warm end, where the cycle repeats. Aheat pipe requires no energy input. For cooling, the process isreversed by tilting the pipe the other way.

Controls may be pneumatic, electric, electronic, digital, ora combination. Satisfactory control can be achieved usingindependent control loops on each system. Maximum operatingefficiency and comfort levels can be achieved with a controlsystem which adjusts the central system operation to thedemands of the zones. Such a system can save enough inoperating costs to pay for itself in a short time.

Controls for the air handling system and zones arespecifically designed for a building by the architect, engineer,or team who designs the building. The controls are usuallyinstalled at the job site. Terminal unit controls are typicallyfactory installed. Boilers, heat pumps, and rooftop units areusually sold with a factory-installed control packagespecifically designed for that unit.

COOLING

GENERAL

Both sensible and latent heat contribute to the cooling loadof a building. Heat gain is sensible when heat is added to theconditioned space. Heat gain is latent when moisture is addedto the space (e.g., by vapor emitted by occupants and othersources). To maintain a constant humidity ratio in the space,water vapor must be removed at a rate equal to its rate ofaddition into the space.

Conduction is the process by which heat moves betweenadjoining spaces with unequal space temperatures. Heat maymove through exterior walls and the roof, or through floors,walls, or ceilings. Solar radiation heats surfaces which thentransfer the heat to the surrounding air. Internal heat gain isgenerated by occupants, lighting, and equipment. Warm airentering a building by infiltration and through ventilation alsocontributes to heat gain.

Building orientation, interior and exterior shading, the angleof the sun, and prevailing winds affect the amount of solarheat gain, which can be a major source of heat. Solar heatreceived through windows causes immediate heat gain. Areaswith large windows may experience more solar gain in winterthan in summer. Building surfaces absorb solar energy, becomeheated, and transfer the heat to interior air. The amount ofchange in temperature through each layer of a compositesurface depends on the resistance to heat flow and thicknessof each material.

Occupants, lighting, equipment, and outdoor air ventilationand infiltration requirements contribute to internal heat gain.For example, an adult sitting at a desk produces about 400 Btuper hour. Incandescent lighting produces more heat thanfluorescent lighting. Copiers, computers, and other officemachines also contribute significantly to internal heat gain.



12

COOLING EQUIPMENT

An air handling system cools by moving air across a coilcontaining a cooling medium (e.g., chilled water or arefrigerant). Figures 10 and 11 show air handling systems thatuse a chilled water coil and a refrigeration evaporator (directexpansion) coil, respectively. Chilled water control is usuallyproportional, whereas control of an evaporator coil is two-position. In direct expansion systems having more than onecoil, a thermostat controls a solenoid valve for each coil andthe compressor is cycled by a refrigerant pressure control. Thistype of system is called a “pump down” system. Pump downmay be used for systems having only one coil, but more oftenthe compressor is controlled directly by the thermostat.

TEMPERATURECONTROLLER

SENSOR

CONTROLVALVE

CHILLED WATERSUPPLY

CHILLEDWATERCOIL

COOL AIR

CHILLED WATER RETURN

C2707-2

Fig. 10. System Using Cooling Coil.

D

X

TEMPERATURECONTROLLER SENSOR

COOL AIR

C2708-1

EVAPORATORCOIL

SOLENOIDVALVE

REFRIGERANTLIQUID

REFRIGERANTGAS

Fig. 11. System Using Evaporator(Direct Expansion) Coil.

Two basic types of cooling systems are available: chillers,typically used in larger systems, and direct expansion (DX)coils, typically used in smaller systems. In a chiller, therefrigeration system cools water which is then pumped to coilsin the central air handling system or to the coils of fan coilunits, a zone system, or other type of cooling system. In a DXsystem, the DX coil of the refrigeration system is located inthe duct of the air handling system. Condenser cooling forchillers may be air or water (using a cooling tower), while DXsystems are typically air cooled. Because water cooling is moreefficient than air cooling, large chillers are always water cooled.

Compressors for chilled water systems are usuallycentrifugal, reciprocating, or screw type. The capacities ofcentrifugal and screw-type compressors can be controlled byvarying the volume of refrigerant or controlling the compressorspeed. DX system compressors are usually reciprocating and,in some systems, capacity can be controlled by unloadingcylinders. Absorption refrigeration systems, which use heatenergy directly to produce chilled water, are sometimes usedfor large chilled water systems.

While heat pumps are usually direct expansion, a large heatpump may be in the form of a chiller. Air is typically the heatsource and heat sink unless a large water reservoir (e.g., groundwater) is available.

Initial and operating costs are prime factors in selectingcooling equipment. DX systems can be less expensive thanchillers. However, because a DX system is inherently two-position (on/off), it cannot control temperature with theaccuracy of a chilled water system. Low-temperature controlis essential in a DX system used with a variable air volumesystem.

For more information control of various system equipment,refer to the following sections of this manual:

— Chiller, Boiler, and Distribution SystemControl Application.

— Air Handling System Control Applications.— Individual Room Control Applications.

DEHUMIDIFICATION

Air that is too humid can cause problems such ascondensation and physical discomfort. Dehumidificationmethods circulate moist air through cooling coils or sorptionunits. Dehumidification is required only during the coolingseason. In those applications, the cooling system can bedesigned to provide dehumidification as well as cooling.

For dehumidification, a cooling coil must have a capacityand surface temperature sufficient to cool the air below its dewpoint. Cooling the air condenses water, which is then collectedand drained away. When humidity is critical and the coolingsystem is used for dehumidification, the dehumidified air maybe reheated to maintain the desired space temperature.

When cooling coils cannot reduce moisture contentsufficiently, sorption units are installed. A sorption unit useseither a rotating granular bed of silica gel, activated aluminaor hygroscopic salts (Fig. 12), or a spray of lithium chloridebrine or glycol solution. In both types, the sorbent materialabsorbs moisture from the air and then the saturated sorbentmaterial passes through a separate section of the unit thatapplies heat to remove moisture. The sorbent material givesup moisture to a stream of “scavenger” air, which is thenexhausted. Scavenger air is often exhaust air or could beoutdoor air.



13

Fig. 12. Granular Bed Sorption Unit.

Sprayed cooling coils (Fig. 13) are often used for spacehumidity control to increase the dehumidifier efficiency andto provide year-round humidity control (winter humidificationalso).

DRY AIR

HUMIDAIR

ROTATINGGRANULARBED

SORPTIONUNIT

SCAVENGERAIR

HEATINGCOIL

HUMID AIREXHAUST

C2709

MOISTUREELIMINATORS

SPRAYPUMP M10511

COOLINGCOIL

Fig. 13. Sprayed Coil Dehumidifier.

For more information on dehumidification, refer to thefollowing sections of this manual:

— Psychrometric Chart Fundamentals.— Air Handling System Control Applications.

HUMIDIFICATION

Low humidity can cause problems such as respiratorydiscomfort and static electricity. Humidifiers can humidify aspace either directly or through an air handling system. Forsatisfactory environmental conditions, the relative humidityof the air should be 30 to 60 percent. In critical areas whereexplosive gases are present, 50 percent minimum isrecommended. Humidification is usually required only duringthe heating season except in extremely dry climates.

Humidifiers in air handling systems typically inject steamdirectly into the air stream (steam injection), spray atomizedwater into the air stream (atomizing), or evaporate heated waterfrom a pan in the duct into the air stream passing through theduct (pan humidification). Other types of humidifiers are awater spray and sprayed coil. In spray systems, the water canbe heated for better vaporization or cooled fordehumidification.

For more information on humidification, refer to thefollowing sections of this manual:

— Psychrometric Chart Fundamentals.— Air Handling System Control Applications.

VENTILATION

Ventilation introduces outdoor air to replenish the oxygensupply and rid building spaces of odors and toxic gases.Ventilation can also be used to pressurize a building to reduceinfiltration. While ventilation is required in nearly all buildings,the design of a ventilation system must consider the cost ofheating and cooling the ventilation air. Ventilation air must bekept at the minimum required level except when used for freecooling (refer to ASHRAE Standard 62, Ventilation forAcceptable Indoor Air Quality).

To ensure high-quality ventilation air and minimize theamount required, the outdoor air intakes must be located toavoid building exhausts, vehicle emissions, and other sourcesof pollutants. Indoor exhaust systems should collect odors orcontaminants at their source. The amount of ventilation abuilding requires may be reduced with air washers, highefficiency filters, absorption chemicals (e.g., activatedcharcoal), or odor modification systems.

Ventilation requirements vary according to the number ofoccupants and the intended use of the space. For a breakdownof types of spaces, occupancy levels, and required ventilation,refer to ASHRAE Standard 62.

Figure 14 shows a ventilation system that supplies 100percent outdoor air. This type of ventilation system is typicallyused where odors or contaminants originate in the conditionedspace (e.g., a laboratory where exhaust hoods and fans removefumes). Such applications require make-up air that isconditioned to provide an acceptable environment.

EXHAUST

TOOUTDOORS

EXHAUSTFAN

RETURNAIR

SPACE

MAKE-UPAIR

SUPPLY FAN

COILFILTER

OUTDOORAIR

SUPPLY

C2711

Fig. 14. Ventilation System Using 100 PercentOutdoor Air.

In many applications, energy costs make 100 percent outdoorair constant volume systems uneconomical. For that reason,other means of controlling internal contaminants are available,such as variable volume fume hood controls, spacepressurization controls, and air cleaning systems.

A ventilation system that uses return air (Fig. 15) is morecommon than the 100 percent outdoor air system. The return-air ventilation system recirculates most of the return air fromthe system and adds outdoor air for ventilation. The return-airsystem may have a separate fan to overcome duct pressure



14

losses. The exhaust-air system may be incorporated into theair conditioning unit, or it may be a separate remote exhaust.Supply air is heated or cooled, humidified or dehumidified,and discharged into the space.

DAMPER RETURN FAN

RETURNAIR

EXHAUSTAIR

DAMPERS

OUTDOORAIR

MIXEDAIR

FILTER COIL SUPPLY FAN

SUPPLYAIR

C2712

Fig. 15. Ventilation System Using Return Air.

Ventilation systems as shown in Figures 14 and 15 shouldprovide an acceptable indoor air quality, utilize outdoor airfor cooling (or to supplement cooling) when possible, andmaintain proper building pressurization.

For more information on ventilation, refer to the followingsections of this manual:

— Indoor Air Quality Fundamentals.— Air Handling System Control Applications.— Building Airflow System Control Applications.

FILTRATION

Air filtration is an important part of the central air handlingsystem and is usually considered part of the ventilation system.Two basic types of filters are available: mechanical filters andelectrostatic precipitation filters (also called electronic aircleaners). Mechanical filters are subdivided into standard andhigh efficiency.

Filters are selected according to the degree of cleanlinessrequired, the amount and size of particles to be removed, andacceptable maintenance requirements. High-efficiencyparticulate air (HEPA) mechanical filters (Fig. 16) do notrelease the collected particles and therefore can be used forclean rooms and areas where toxic particles are released. HEPAfilters significantly increase system pressure drop, which mustbe considered when selecting the fan. Figure 17 shows othermechanical filters.

C2713

CELL

PLEATED PAPER

AIR FLOW

Fig. 16. HEPA Filter.

PLEATED FILTER

BAG FILTER

Fig. 17. Mechanical Filters.

Other types of mechanical filters include strainers, viscouscoated filters, and diffusion filters. Straining removes particlesthat are larger than the spaces in the mesh of a metal filter andare often used as prefilters for electrostatic filters. In viscouscoated filters, the particles passing through the filter fiberscollide with the fibers and are held on the fiber surface.Diffusion removes fine particles by using the turbulence presentin the air stream to drive particles to the fibers of the filtersurface.

An electrostatic filter (Fig. 18) provides a low pressure dropbut often requires a mechanical prefilter to collect largeparticles and a mechanical after-filter to collect agglomeratedparticles that may be blown off the electrostatic filter. Anelectrostatic filter electrically charges particles passing throughan ionizing field and collects the charged particles on plateswith an opposite electrical charge. The plates may be coatedwith an adhesive.



15

Fig. 18. Electrostatic Filter.

The sensor can be separate from or part of the controllerand is located in the controlled medium. The sensor measuresthe value of the controlled variable and sends the resultingsignal to the controller. The controller receives the sensorsignal, compares it to the desired value, or setpoint, andgenerates a correction signal to direct the operation of thecontrolled device. The controlled device varies the controlagent to regulate the output of the control equipment thatproduces the desired condition.

HVAC applications use two types of control loops: openand closed. An open-loop system assumes a fixed relationshipbetween a controlled condition and an external condition. Anexample of open-loop control would be the control of perimeterradiation heating based on an input from an outdoor airtemperature sensor. A circulating pump and boiler are energizedwhen an outdoor air temperature drops to a specified setting,and the water temperature or flow is proportionally controlledas a function of the outdoor temperature. An open-loop systemdoes not take into account changing space conditions frominternal heat gains, infiltration/exfiltration, solar gain, or otherchanging variables in the building. Open-loop control alonedoes not provide close control and may result in underheatingor overheating. For this reason, open-loop systems are notcommon in residential or commercial applications.

A closed-loop system relies on measurement of thecontrolled variable to vary the controller output. Figure 19shows a block diagram of a closed-loop system. An exampleof closed-loop control would be the temperature of dischargeair in a duct determining the flow of hot water to the heatingcoils to maintain the discharge temperature at a controllersetpoint.

AIRFLOW

AIRFLOW

ALTERNATEPLATESGROUNDED

INTERMEDIATEPLATESCHARGEDTO HIGHPOSITIVEPOTENTIAL

THEORETICALPATHS OFCHARGES DUSTPARTICLESPOSITIVELY CHARGED

PARTICLES

SOURCE: 1996 ASHRAE SYSTEMS AND EQUIPMENT HANDBOOK

PATH OFIONS

WIRES AT HIGHPOSITIVEPOTENTIAL

C2714

–

+

–

–

–

–

+

+

+

CONTROL SYSTEM CHARACTERISTICS

Automatic controls are used wherever a variable conditionmust be controlled. In HVAC systems, the most commonlycontrolled conditions are pressure, temperature, humidity, andrate of flow. Applications of automatic control systems rangefrom simple residential temperature regulation to precisioncontrol of industrial processes.

CONTROLLED VARIABLES

Automatic control requires a system in which a controllablevariable exists. An automatic control system controls thevariable by manipulating a second variable. The secondvariable, called the manipulated variable, causes the necessarychanges in the controlled variable.

In a room heated by air moving through a hot water coil, forexample, the thermostat measures the temperature (controlledvariable) of the room air (controlled medium) at a specifiedlocation. As the room cools, the thermostat operates a valvethat regulates the flow (manipulated variable) of hot water(control agent) through the coil. In this way, the coil furnishesheat to warm the room air.

CONTROL LOOP

In an air conditioning system, the controlled variable ismaintained by varying the output of the mechanical equipmentby means of an automatic control loop. A control loop consistsof an input sensing element, such as a temperature sensor; acontroller that processes the input signal and produces an outputsignal; and a final control element, such as a valve, that operatesaccording to the output signal.



16

Fig. 19. Feedback in a Closed-Loop System.

In this example, the sensing element measures the dischargeair temperature and sends a feedback signal to the controller.The controller compares the feedback signal to the setpoint.Based on the difference, or deviation, the controller issues acorrective signal to a valve, which regulates the flow of hotwater to meet the process demand. Changes in the controlledvariable thus reflect the demand. The sensing element continuesto measure changes in the discharge air temperature and feedsthe new condition back into the controller for continuouscomparison and correction.

Automatic control systems use feedback to reduce themagnitude of the deviation and produce system stability asdescribed above. A secondary input, such as the input from anoutdoor air compensation sensor, can provide informationabout disturbances that affect the controlled variable. Usingan input in addition to the controlled variable enables thecontroller to anticipate the effect of the disturbance andcompensate for it, thus reducing the impact of disturbances onthe controlled variable.

CONTROL METHODS

GENERAL

An automatic control system is classified by the type ofenergy transmission and the type of control signal (analog ordigital) it uses to perform its functions.

The most common forms of energy for automatic controlsystems are electricity and compressed air. Systems maycomprise one or both forms of energy.

Systems that use electrical energy are electromechanical,electronic, or microprocessor controlled. Pneumatic controlsystems use varying air pressure from the sensor as input to acontroller, which in turn produces a pneumatic output signalto a final control element. Pneumatic, electromechanical, andelectronic systems perform limited, predetermined controlfunctions and sequences. Microprocessor-based controllers usedigital control for a wide variety of control sequences.

Self-powered systems are a comparatively minor but stillimportant type of control. These systems use the power of themeasured variable to induce the necessary corrective action.For example, temperature changes at a sensor cause pressureor volume changes that are applied directly to the diaphragmor bellows in the valve or damper actuator.

Many complete control systems use a combination of theabove categories. An example of a combined system is thecontrol system for an air handler that includes electric on/offcontrol of the fan and pneumatic control for the heating andcooling coils.

Various control methods are described in the followingsections of this manual:

— Pneumatic Control Fundamentals.— Electric Control Fundamentals.— Electronic Control Fundamentals.— Microprocessor-Based/DDC Fundamental.

See CHARACTERISTICS AND ATTRIBUTES OFCONTROL METHODS.

ANALOG AND DIGITAL CONTROL

Traditionally, analog devices have performed HVAC control.A typical analog HVAC controller is the pneumatic type whichreceives and acts upon data continuously. In a pneumaticcontroller, the sensor sends the controller a continuouspneumatic signal, the pressure of which is proportional to thevalue of the variable being measured. The controller comparesthe air pressure sent by the sensor to the desired value of airpressure as determined by the setpoint and sends out a controlsignal based on the comparison.

The digital controller receives electronic signals fromsensors, converts the electronic signals to digital pulses(values), and performs mathematical operations on thesevalues. The controller reconverts the output value to a signalto operate an actuator. The controller samples digital data atset time intervals, rather than reading it continually. Thesampling method is called discrete control signaling. If thesampling interval for the digital controller is chosen properly,discrete output changes provide even and uninterrupted controlperformance.

Figure 20 compares analog and digital control signals. Thedigital controller periodically updates the process as a functionof a set of measured control variables and a given set of controlalgorithms. The controller works out the entire computation,including the control algorithm, and sends a signal to anactuator. In many of the larger commercial control systems,an electronic-pneumatic transducer converts the electric outputto a variable pressure output for pneumatic actuation of thefinal control element.

SETPOINT

FEEDBACKCONTROLLER

SECONDARYINPUT

CORRECTIVESIGNAL

FINAL CONTROLELEMENT

PROCESS DISTURBANCES

CONTROLLEDVARIABLESENSING

ELEMENT

MANIPULATEDVARIABLE

C2072



17

Fig. 20. Comparison of Analog and Digital Control Signals.

CONTROL MODES

Control systems use different control modes to accomplishtheir purposes. Control modes in commercial applicationsinclude two-position, step, and floating control; proportional,proportional-integral, and proportional-integral-derivativecontrol; and adaptive control.

TWO-POSITION CONTROL

GENERAL

In two-position control, the final control element occupiesone of two possible positions except for the brief period whenit is passing from one position to the other. Two-position controlis used in simple HVAC systems to start and stop electricmotors on unit heaters, fan coil units, and refrigerationmachines, to open water sprays for humidification, and toenergize and deenergize electric strip heaters.

In two-position control, two values of the controlled variable(usually equated with on and off) determine the position ofthe final control element. Between these values is a zone calledthe “differential gap” or “differential” in which the controllercannot initiate an action of the final control element. As thecontrolled variable reaches one of the two values, the finalcontrol element assumes the position that corresponds to thedemands of the controller, and remains there until the controlledvariable changes to the other value. The final control elementmoves to the other position and remains there until thecontrolled variable returns to the other limit.

An example of differential gap would be in a cooling systemin which the controller is set to open a cooling valve when thespace temperature reaches 78F, and to close the valve whenthe temperature drops to 76F. The difference between the twotemperatures (2 degrees F) is the differential gap. Thecontrolled variable fluctuates between the two temperatures.

Basic two-position control works well for many applications.For close temperature control, however, the cycling must beaccelerated or timed.

BASIC TWO-POSITION CONTROL

In basic two-position control, the controller and the finalcontrol element interact without modification from amechanical or thermal source. The result is cyclical operationof the controlled equipment and a condition in which thecontrolled variable cycles back and forth between two values(the on and off points) and is influenced by the lag in thesystem. The controller cannot change the position of the finalcontrol element until the controlled variable reaches one orthe other of the two limits of the differential. For that reason,the differential is the minimum possible swing of the controlledvariable. Figure 21 shows a typical heating system cyclingpattern.

TEMPERATURE(°F)

OFF

ON

75

74

73

72

71

70

69

68

TIME

UNDERSHOOTCONDTION

DIFFERENTIAL

DIAL SETTING

OVERSHOOT CONDTION

C2088

Fig. 21. Typical Operation of Basic Two-Position Control.

The overshoot and undershoot conditions shown in Figure21 are caused by the lag in the system. When the heating systemis energized, it builds up heat which moves into the space towarm the air, the contents of the space, and the thermostat. Bythe time the thermostat temperature reaches the off point (e.g.,72F), the room air is already warmer than that temperature.When the thermostat shuts off the heat, the heating systemdissipates its stored heat to heat the space even more, causingovershoot. Undershoot is the same process in reverse.

In basic two-position control, the presence of lag causes thecontroller to correct a condition that has already passed ratherthan one that is taking place or is about to take place.Consequently, basic two-position control is best used insystems with minimal total system lag (including transfer,measuring, and final control element lags) and where closecontrol is not required.

ANALOG CONTROL SIGNAL

DIGITAL CONTROL SIGNAL

OPEN

FINALCONTROLELEMENTPOSITION

CLOSED

OPEN

FINALCONTROLELEMENTPOSITION

CLOSED

TIME

TIME C2080



18

Figure 22 shows a sample control loop for basic two-positioncontrol: a thermostat turning a furnace burner on or off inresponse to space temperature. Because the thermostat cannotcatch up with fluctuations in temperature, overshoot andundershoot enable the temperature to vary, sometimesconsiderably. Certain industrial processes and auxiliaryprocesses in air conditioning have small system lags and canuse two-position control satisfactorily.

Fig. 22. Basic Two-Position Control Loop.

TIMED TWO-POSITION CONTROL

GENERAL

The ideal method of controlling the temperature in a spaceis to replace lost heat or displace gained heat in exactly theamount needed. With basic two-position control, such exactoperation is impossible because the heating or cooling systemis either full on or full off and the delivery at any specific instantis either too much or too little. Timed two-position control,however, anticipates requirements and delivers measuredquantities of heating or cooling on a percentage on-time basisto reduce control point fluctuations. The timing is accomplishedby a heat anticipator in electric controls and by a timer inelectronic and digital controls.

In timed two-position control, the basic interaction betweenthe controller and the final control element is the same as forbasic two-position control. However, the controller respondsto gradual changes in the average value of the controlledvariable rather than to cyclical fluctuations.

Overshoot and undershoot are reduced or eliminated becausethe heat anticipation or time proportioning feature results in afaster cycling rate of the mechanical equipment. The result iscloser control of the variable than is possible in basic two-position control (Fig. 23).

Fig. 23. Comparison of Basic Two-Position and TimedTwo-Position Control.

HEAT ANTICIPATION

In electromechanical control, timed two-position control canbe achieved by adding a heat anticipator to a bimetal sensingelement. In a heating system, the heat anticipator is connectedso that it energizes whenever the bimetal element calls for heat.On a drop in temperature, the sensing element acts to turn onboth the heating system and the heat anticipator. The heatanticipator heats the bimetal element to its off point early anddeenergizes the heating system and the heat anticipator. Asthe ambient temperature falls, the time required for the bimetalelement to heat to the off point increases, and the cooling timedecreases. Thus, the heat anticipator automatically changesthe ratio of on time to off time as a function of ambienttemperature.

Because the heat is supplied to the sensor only, the heatanticipation feature lowers the control point as the heatrequirement increases. The lowered control point, called“droop”, maintains a lower temperature at design conditionsand is discussed more thoroughly in the following paragraphs.Energizing the heater during thermostat off periodsaccomplishes anticipating action in cooling thermostats. Ineither case, the percentage on-time varies in proportion to thesystem load.

THERMOSTAT

FURNACE

SOLENOIDGAS VALVE

C2715

72

71

73

70

69

74

75

68

CONTROLPOINT

TIME C2089

TIMED TWO-POSITION CONTROL

72

71

73

70

69

74

75

68

OFF

ON

TEMPERATURE(°F)

DIFFERENTIAL

DIAL SETTING

UNDERSHOOTCONDITION

TIME

OVERSHOOTCONDITION

BASIC TWO-POSITION CONTROL

TEMPERATURE(°F)

OFF

ON



19

TIME PROPORTIONING

Time proportioning control provides more effective two-position control than heat anticipation control and is availablewith some electromechanical thermostats and in electronic andmicroprocessor-based controllers. Heat is introduced into thespace using on/off cycles based on the actual heat load on thebuilding and programmable time cycle settings. This methodreduces large temperature swings caused by a large total lagand achieves a more even flow of heat.

In electromechanical thermostats, the cycle rate is adjustableby adjusting the heater. In electronic and digital systems, thetotal cycle time and the minimum on and off times of thecontroller are programmable. The total cycle time setting isdetermined primarily by the lag of the system under control.If the total cycle time setting is changed (e.g., from 10 minutesto 20 minutes), the resulting on/off times change accordingly(e.g., from 7.5 minutes on/2.5 minutes off to 15 minutes on/5minutes off), but their ratio stays the same for a given load.

The cycle time in Figure 24 is set at ten minutes. At a 50percent load condition, the controller, operating at setpoint,produces a 5 minute on/5 minute off cycle. At a 75 percentload condition, the on time increases to 7.5 minutes, the offtime decreases to 2.5 minutes, and the opposite cycle ratiooccurs at 25 percent load. All load conditions maintain thepreset 10-minute total cycle.10

7.5

5

2.5

0

SELECTEDCYCLE TIME(MINUTES)

100 75 50 25 0

LOAD (%)

ON

OFF

C2090

73-1/4

72

70-3/4

0

1-1/4

2-1/2

DR

OO

P (°F

)

CO

NT

RO

L P

OIN

T (°F

)

DESIGNTEMPERATURE

OUTDOOR AIRTEMPERATURE

0% 100%LOADC2091-1

NO LOADTEMPERATURE

Fig. 24. Time Proportioning Control.

Because the controller responds to average temperature orhumidity, it does not wait for a cyclic change in the controlledvariable before signaling corrective action. Thus control systemlags have no significant effect.

Droop in heating control is a lowering of the control pointas the load on the system increases. In cooling control, droopis a raising of the control point. In digital control systems,droop is adjustable and can be set as low as one degree or evenless. Figure 25 shows the relationship of droop to load.

Fig. 25. Relationship between Control Point, Droop,and Load (Heating Control).

Time proportioning control of two-position loads isrecommended for applications such as single-zone systems thatrequire two-position control of heating and/or cooling (e.g., agas-fired rooftop unit with direct-expansion cooling). Timeproportioning control is also recommended for electric heatcontrol, particularly for baseboard electric heat. With timeproportioning control, care must be used to avoid cycling thecontrolled equipment more frequently than recommended bythe equipment manufacturer.

STEP CONTROL

Step controllers operate switches or relays in sequence toenable or disable multiple outputs, or stages, of two-positiondevices such as electric heaters or reciprocating refrigerationcompressors. Step control uses an analog signal to attempt toobtain an analog output from equipment that is typically eitheron or off. Figures 26 and 27 show that the stages may bearranged to operate with or without overlap of the operating(on/off) differentials. In either case, the typical two-positiondifferentials still exist but the total output is proportioned.

74

ONOFF

ONOFF

ONOFF

ONOFF

ONOFF

5

4

3

2

1

DIFFERENTIAL

THROTTLING RANGE

72SPACE TEMPERATURE (°F)

100%0% LOADC2092-1

STAGES

Fig. 26. Electric Heat Stages.



20

Fig. 27. Staged Reciprocating Chiller Control.

Figure 28 shows step control of sequenced DX coils andelectric heat. On a rise in temperature through the throttlingrange at the thermostat, the heating stages sequence off. On afurther rise after a deadband, the cooling stages turn on insequence.

ACTUATOR

AIRFLOW

DAMPER

REFERENCEPRESSURE

PICK-UP

STATICPRESSURE

PICK-UP

FLOATINGSTATIC

PRESSURECONTROLLER

C2717

zero, and the sequence repeats until all stages required to meetthe load condition are on. On a decrease in load, the processreverses.

With microprocessor controls, step control is usually donewith multiple, digital, on-off outputs since software allowseasily adjustable on-to-off per stage and interstage differentialsas well as no-load and time delayed startup and minimum onand off adjustments.

FLOATING CONTROL

Floating control is a variation of two-position control and isoften called “three-position control”. Floating control is not acommon control mode, but is available in most microprocessor-based control systems.

Floating control requires a slow-moving actuator and a fast-responding sensor selected according to the rate of responsein the controlled system. If the actuator should move too slowly,the controlled system would not be able to keep pace withsudden changes; if the actuator should move too quickly, two-position control would result.

Floating control keeps the control point near the setpoint atany load level, and can only be used on systems with minimallag between the controlled medium and the control sensor.Floating control is used primarily for discharge control systemswhere the sensor is immediately downstream from the coil,damper, or device that it controls. An example of floatingcontrol is the regulation of static pressure in a duct (Fig. 29).

ONOFF

ONOFF

ONOFF

ONOFF

4

3

2

1

DIFFERENTIAL

THROTTLING RANGE

76

SPACE TEMPERATURE (°F)

72

100%0% LOADC2093

STAGES

74SETPOINT

SPACE ORRETURN AIR

THERMOSTATACTUATOR

SOLENOIDVALVES

FAN

DISCHARGEAIR

DIRECT EXPANSIONCOILS

MULTISTAGEELECTRIC HEAT

STEPCONTROLLER

STAGE NUMBERS6

5

4

3

2

1

C2716

D

X

D

X

Fig. 28. Step Control with Sequenced DX Coils andElectric Heat.

A variation of step control used to control electric heat isstep-plus-proportional control, which provides a smoothtransition between stages. This control mode requires one ofthe stages to be a proportional modulating output and the others,two-position. For most efficient operation, the proportionalmodulating stage should have at least the same capacity asone two-position stage.

Starting from no load, as the load on the equipment increases,the modulating stage proportions its load until it reaches fulloutput. Then, the first two-position stage comes full on andthe modulating stage drops to zero output and begins toproportion its output again to match the increasing load. Whenthe modulating stage again reaches full output, the second two-position stage comes full on, the modulating stage returns to

Fig. 29. Floating Static Pressure Control.

In a typical application, the control point moves in and outof the deadband, crossing the switch differential (Fig. 30). Adrop in static pressure below the controller setpoint causes theactuator to drive the damper toward open. The narrowdifferential of the controller stops the actuator after it has moveda short distance. The damper remains in this position until thestatic pressure further decreas

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HONEYWELL E M AUTOMATIC CONTROL for - Homestead€¦ · tremendous advances in equipment, system design, and application. In this edition, microprocessor controls are shown in most